1. Field of the Invention
The invention relates to the manufacture of plastics material parts having at least one mechanically weakened area forming a hole or a hole precursor.
It is directed in particular, although not exclusively, to the manufacture of plastics material sections incorporating a longitudinal succession of holes or hole precursors intended to enable easy fixing of the sections in use. These sections are advantageously sections constituting or forming part of electrical wiring trunking.
As used herein the expression xe2x80x9chole precursorxe2x80x9d (the concept of a xe2x80x9cpreformxe2x80x9d could equally well be used) means a hole that is imperfectly formed, constituted by an area that is mechanically weakened relative to the remainder of the part concerned; a mechanically weakened area of this kind can in particular be a hole formed in only part of the local thickness of the part concerned, a hole having over a portion of its depth a web including a constriction of its cross section which is generally concentric with the hole and intended to be destroyed when a fixing member is inserted, or a mechanically weakened contour separating from the remainder of the part a central portion which is intended to be eliminated in use; the aforementioned features can be combined, for example, with a blind hole including a cross section constriction and the bottom of which has a mechanically weakened contour. The mechanically weakened area can have an oblong section and can constitute a hole with a spot facing.
The benefit of providing a hole precursor in the form of a blind hole, rather than a through-hole, is that the user is free to use only some of the hole precursors to fix the part concerned, for example, without the unused precursors constituting traps for impurities or dust or even compromising the sealing of the internal volume of the part; this benefit is particularly significant in trunking, for example electrical wiring trunking, which is required to be as well sealed as possible from the external environment (this applies in particular to electrical wiring trunking intended for use in wet areas in the home).
If the hole precursor is open, the seal is not preserved so well, but the existence of constrictions in the hole precursors facilitates centering fixing members (usually screws) in the hole precursors which are used in service.
The fact of providing hole precursors constituted of mechanically weakened contours is that the areas of the parts containing the hole precursors that are not used in service are more rigid than if the holes were formed completely, which is combined with facilitating the fitting of fixing members into the precursors that are actually used.
The benefit of oblong hole precursors is that they can have a cross section larger than the cross section of the fixing members eventually used and enable the user to choose the location of the fixing hole within the oblong precursor.
The benefit of spot facings (recessed bearing surfaces which are permanent) is well known; in particular, it enables the head of a fixing screw to be accommodated within the thickness of the part.
2. Description of the Prior Art
The skilled person knows how to make hole precursors or holes with spot facings in plastics material parts. In particular, the skilled person knows how to manufacture parts by injection molding with fixed projections or more usually mobile slides incorporated into the design of the mold parts. However, this approach, whereby the spot-faced holes or the hole precursors are formed as soon as the plastic material is shaped, makes it necessary to design the molds accordingly and is applicable only to injection molded parts whose geometry allows this.
The object of the invention is to provide a method of manufacturing plastics material parts with mechanically weakened areas forming holes or hole precursors (in practice a greater number of such holes or hole precursors than is necessary for using the parts), wherein the mechanically weakened areas are formed esthetically and easily, at moderate cost, after the parts (or at least blanks therefor) are produced, and in a manner that is both reproducible and reliable.
To this end, the invention proposes a method of manufacturing a part having at least one mechanically weakened area, the method including a step of producing a thermoplastics material part and at least one cycle of steps entailing forming a localized opening through an area of the part, heating the part locally by means of a heating probe so as to render the area of the part plastic, and using a punch to modify the geometry of the area rendered plastic so as to delimit in the part a mechanically weakened area constituting a hole or hole precursor.
Thus, in accordance with the invention, the mechanically weakened area is conformed by deforming a plastic area of the part using a punch whose shape determines the shape of the mechanically weakened area after cooling. The punch causes plastic flow of the material, in practice against a counter-punch which can be flush with the surface of the part on the opposite side to the punch. Thus the part is not punched right through but through only a part of its thickness, there remaining material afterwards in line with the punch (considering its largest cross section).
The benefit of the opening is in particular to avoid the creation of an increase in thickness on producing the mechanically weakened area as the result of displacement of material during punching; the fact that the opening is a through-opening (with a section that in practice is constant) corresponds to a simple and reproducible manner of forming a opening.
For the plastics material to flow in a homogeneous manner, the area rendered plastic is preferably formed around the opening.
The hole precursor can have several conformations.
In a first conformation, the geometry of the area rendered plastic is modified to delimit the mechanically weakened area in the form of a blind hole the bottom of which is constituted of a continuous web. This web guarantees that unused hole precursors are sealed. The web is preferably flush with the surface of the part on the side opposite the punch, in which case it is not necessary to provide a counter-punch of particular shape (a simple plane surface can be used).
In another, advantageous conformation, the geometry of the area rendered plastic is modified to delimit the mechanically weakened area in the form of a through-hole bordered by a peripheral web. The peripheral web may be understood as an incomplete web (in the sense in which this term is used in connection with the first conformation referred to above). It materializes a cross section constriction which is advantageously centered on the remainder of the hole precursor and can be used to center correctly the tool for preparing a fixing hole in the wall to which the part must be fixed, or even the fixing member itself. Depending on its thickness, either the peripheral web is destroyed when fitting a fixing member or remains and constitutes a spot facing.
In a further, advantageous conformation, the geometry of the area rendered plastic is modified to delimit the mechanically weakened area by a contour made up of openings. This corresponds to forming xe2x80x9cdashed linesxe2x80x9d of openings around the contour of a hole obtained by breaking the residual areas of material linking the inside and the outside of the contour. The contour is preferably formed of through-openings, which facilitates breaking the residual connecting areas. However, the openings materializing the contour can instead be blind openings. In a further variant, the contour can be a contour of constant thickness less than the local thickness of the part (like a groove).
The method of the invention is of particular benefit if the thermoplastics material part is extruded. This is because the extrusion process produces a section of constant cross section and forming hole precursors necessarily requires specific operations.
Mechanically weakened areas can be produced after the extrusion process, exploiting the fact that the material of the section is still warm: the input of heat (or more generally of energy) to soften the plastics material locally can be reduced commensurately. To this end, the cycle of steps is advantageously carried out as the product is extruded. These steps can instead be carried out on sections previously cut to length.
The method of the invention is all the more beneficial when there is potentially a succession of hole precursors in the part; to this end, the cycle of steps is preferably reiterated to form a succession of hole precursors.
In a particularly advantageous embodiment of the invention the part is locally heated by means of a probe having one portion which constitutes the punch for modifying the geometry of the area rendered plastic. This represents the optimum from the thermal point of view (there are no thermal losses between the time at which the material is locally heated to the maximum temperature and the time at which the punch is applied), and the number of moving parts is minimized.
It is particularly advantageous if the heating probe is an ultrasound probe (sometimes referred to as a xe2x80x9csonotrodexe2x80x9d).
As already mentioned the shape of the punch determines that of the future hole precursor or hole.
A first embodiment of the punch has an end portion terminating in a plane transverse face; this produces a web at the bottom of the hole precursor whose thickness is in theory substantially constant.
A second embodiment of the punch has an end portion terminating in a transverse face incorporating at least one step. This provides a choice of a web closing off the whole of the future hole, with variations of thickness, or a through-hole precursor, forming a constriction bordered by a peripheral web or a hole bordered by a spot facing; the spot facing can itself be closed off by a web which is adapted to be broken.
The end portion can simply have a cylindrical cross section.
It can instead have an oblong cross section; in the case of a section, the punch is preferably oriented so that its greatest dimension is parallel to the longitudinal axis of the section; however, to give the maximum choice as to the location of the fixing members in the hole precursors, the orientation of the punch can be changed between forming two consecutive holes or hole precursors.
The end portion can simply have a constant cross section, but instead it can have a decreasing section; this produces holes or hole precursors which are easily penetrated by a tool or a fixing member from the side on which the hole or hole precursor is wider, without such insertion being as easy from the other side, or even possible.
In a further embodiment the end portion can terminate in a transverse (possibly concave) face bordered by a series of axial teeth extending in a closed line corresponding to the contour to be formed; it can instead terminate in a transverse face bordered by a continuous axial rim.
The invention also covers sections adapted to be made by the aforementioned method. It also provides a plastics material section incorporating a longitudinal succession of mechanically weakened areas forming holes or hole precursors.
Each mechanically weakened area is preferably in the form of a blind hole the bottom of which is constituted by a continuous web, which is favorable to obtaining a good seal between the two faces of the part.
In another embodiment each mechanically weakened area is in the form of a through-hole bordered by a peripheral web or spot facing, which can be of benefit from the centering point of view (even if its cross section is not circular, as the centering effect can be in the longitudinal direction only or in the transverse direction only).
In a further embodiment of the section each mechanically weakened area is delimited by a contour made up of a succession of openings.
The invention also covers the particular situation of electrical wiring trunking including a base portion and a cover, the base portion forming part of a section of the aforementioned type.
Objects, features and advantages of the invention will emerge from the following description which is given by way of illustrative and non-limiting example and with reference to the accompanying drawings.